In electrical circuit testing for continuity of conductive paths on electronic devices, probing of contacts to meter and read voltages, or other related electronic data from electronic devices, it is usual to use a probe to contact the conductive path and thus provide the input contact and path to the appropriate measuring or recording instrumentation. When probing by hand, the force which is applied to the probe neither is controlled nor constant, and the force variations often create measurement errors due to the varying contact resistance between the probe tip and the electrical circuit element. Moreover, when the instrumentation is automated using robotics to position the probes, spring loaded pins are often used to compensate for over-travel of the robotic device which occurs due to the lack of positioning accuracy by the robotic device or due to height variations in the test specimen. When the robotic device positions the probe and if the probe housing position is too close to the test specimen, excessive force may be exerted on the test specimen which may damage either the probe or the test specimen. If the robotic device does not position the probe housing closely enough to the test specimen, sufficient engagement force to assure adequate conductivity between the tip and specimen may not occur.
Measurement errors can be a result of the variance of the contact resistance or, more particularly, may be due to the variances of the forces exerted by any springs of the spring-type pins used to force the probe tip against the test specimen. Spring pins are forced against the specimen with a force that is a function of displacement of the pin and the spring constants, and may engage the specimen with a force which varies widely.
Attempts have been made in the past to compensate for the forces exerted on probe tips in many different environments. An example of a probe tip that is supported on a cantilevered parallel spring beam arrangement is U.S. Pat. No. 5,012,591 to Asakawa, and assigned to Fujitsu, Ltd. The above patent discloses that the deflection of the springs is detected by a spring gauge which controls the DC motor used to position the probe head and a voice coil motor to compensate for the forces generated by the beam springs.
U.S. Pat. No. 5,040,306 to McMurtry et al., and assigned to Renishaw plc., discloses a probe mechanism for measuring the profile of an object, and the probe tip is positioned in contact with the work piece. The probe shaft is translated within the probe housing and a force exerted upon it by a motor. The displacement of the probe tip deflects supporting springs, and the force of the springs is detected as a function of the displacement of the probe tip by a transducer. This data is then used to calculate the information relating to the spatial position of the contact point between the probe tip and the object being measured.
The present invention utilizes magnetic repulsion, which is yielding and compliant, to avoid damage to the probe while at the same time permits control of the force exerted on the probe by the magnetic repulsion; the force which the probe exerts on the test specimen insures that the test specimen and probe are not damaged while adequately engaged for reliable test measurements.
The probe tip is mounted on a magnetic member which is repelled by an electromagnet to exert the desired force level on the test specimen. The control of the force insures that a consistent force of engagement is applied at each test site for damage control and/or consistent test results. The force level is controlled by the amount of electrical current permitted to pass through the coils of the electromagnet. A force sensing device such as a load cell is used to provide an indication of the force exerted on it by the electromagnet being repelled from the magnetic plunger of the probe structure. The output signal of the load cell is converted to digital information, compared with preset values representing desired loads, and adjusted through a digital-to-analog converter and a Darlington transistor network to control the current from the power supply to the coil windings of the electromagnet within the probe housing.
The subject probe and its control may be better understood from references to the drawings and to the detailed description to follow.